There are parasitic (unavoidable) NPNs and PNPs on the output driver of a CMOS chip which are connected in a positive feedback loop that resembles an SCR. If the output swings too far below ground, or too far above the positive supply, the SCR can be triggered, latching up and causing destructive current to flow.http://www.irf.com/technical-info/designtp/dt94-9.pdf